Previously, when a VERSION file exists it takes precendence over the
git version. This is unfortunate because all restic binaries compiled
from a git checkout will just identify as the latest release (e.g.
'0.1.0'), regardeless of any commits on top of it.
This commit adds a combined version string by using the contents of
the VERSION file, and append the current git version returned by `git
describe` if available, e.g.:
0.1.0 (v0.1.0-6-gb188217-dirty).